The fbpA/sapM double knock out strain of Mycobacterium tuberculosis is highly attenuated and immunogenic in macrophages

The SapM phosphatase can arrest phagosome maturation in an ESX-1 independent manner in Mycobacterium tuberculosis and BCG

Mycobacterium tuberculosis (Mtb) is an intracellular pathogen that survives and grows in macrophages. A mechanism used by Mtb to achieve intracellular survival is to secrete effector molecules that arrest the normal process of phagosome maturation. Through phagosome maturation arrest (PMA), Mtb remains in an early phagosome and avoids delivery to degradative phagolysosomes. One PMA effector of Mtb is the secreted SapM phosphatase. Because the host target of SapM, phosphatidylinositol-3-phosphate (PI3P), is located on the cytosolic face of the phagosome, SapM needs to not only be released by the mycobacteria but also travel out of the phagosome to carry out its function. To date, the only mechanism known for Mtb molecules to leave the phagosome is phagosome permeabilization by the ESX-1 secretion system. To understand this step of SapM function in PMA, we generated identical in-frame sapM mutants in both the attenuated Mycobacterium bovis bacille Calmette-Guérin (BCG) vaccine strain, which lacks the ESX-1 system, and Mtb. Characterization of these mutants demonstrated that SapM is required for PMA in BCG and Mtb. Further, by establishing a role for SapM in PMA in BCG, and subsequently in a Mtb mutant lacking the ESX-1 system, we demonstrated that the role of SapM does not require ESX-1. We further determined that ESX-2 or ESX-4 is also not required for SapM to function in PMA. These results indicate that SapM is a secreted effector of PMA in both BCG and Mtb, and that it can function independent of the known mechanism for Mtb molecules to leave the phagosome.

Exploring host-pathogen interactions in the Dictyostelium discoideum-Mycobacterium marinum infection model of tuberculosis

Mycobacterium tuberculosis is a pathogenic mycobacterium that causes tuberculosis. Tuberculosis is a significant global health concern that poses numerous clinical challenges, particularly in terms of finding effective treatments for patients. Throughout evolution, host immune cells have developed cell-autonomous defence strategies to restrain and eliminate mycobacteria. Concurrently, mycobacteria have evolved an array of virulence factors to counteract these host defences, resulting in a dynamic interaction between host and pathogen. Here, we review recent findings, including those arising from the use of the amoeba Dictyostelium discoideum as a model to investigate key mycobacterial infection pathways. D. discoideum serves as a scalable and genetically tractable model for human phagocytes, providing valuable insights into the intricate mechanisms of host-pathogen interactions. We also highlight certain similarities between M. tuberculosis and Mycobacterium marinum, and the use of M. marinum to more safely investigate mycobacteria in D. discoideum.

The ΔfbpAΔsapM candidate vaccine derived from Mycobacterium tuberculosis H37Rv is markedly immunogenic in macrophages and induces robust immunity to tuberculosis in mice

Tuberculosis (TB) remains a significant global health challenge, with approximately 1.5 million deaths per year. The Bacillus Calmette-Guérin (BCG) vaccine against TB is used in infants but shows variable protection. Here, we introduce a novel approach using a double gene knockout mutant (DKO) from wild-type Mycobacterium tuberculosis (Mtb) targeting fbpA and sapM genes. DKO exhibited enhanced anti-TB gene expression in mouse antigen-presenting cells, activating autophagy and inflammasomes. This heightened immune response improved ex vivo antigen presentation to T cells. Subcutaneous vaccination with DKO led to increased protection against TB in wild-type C57Bl/6 mice, surpassing the protection observed in caspase 1/11-deficient C57Bl/6 mice and highlighting the critical role of inflammasomes in TB protection. The DKO vaccine also generated stronger and longer-lasting protection than the BCG vaccine in C57Bl/6 mice, expanding both CD62L-CCR7-CD44+/-CD127+ effector T cells and CD62L+CCR7+/-CD44+CD127+ central memory T cells. These immune responses correlated with a substantial ≥ 1.7-log10 reduction in Mtb lung burden. The DKO vaccine represents a promising new approach for TB immunization that mediates protection through autophagy and inflammasome pathways.

Mycobacterial SapM hampers host autophagy initiation for intracellular bacillary survival via dephosphorylating Raptor

Secreted acid phosphatase (SapM) is an immunomodulator of Mycobacterium tuberculosis (Mtb) and consequently plays a crucial role in disease onset and development upon infection. Importantly, the virulence of SapM has rendered SapM an attractive target for drug development. However, the mechanism underlying the role of SapM in facilitating bacillary survival remains to be fully elucidated. In this context, the present study demonstrated that SapM hampered cellular autophagy to facilitate bacillary survival in mycobacterial-infected macrophages. Mechanically, SapM interacted with Raptor and was localized to the subcellular lysosomal organelle, causing the dephosphorylation of Raptor at the Ser792 position, resulting in mTORC1 hyperactivity and the subsequent autophagy inhibition. Consistent with this, SapM blocked the autophagy initiation and mitigated lung pathology in vivo. These findings highlighted the role of Raptor as a significant substrate of SapM for inhibiting autophagy, which is a novel clue for developing a treatment against tuberculosis.

Live Attenuated Vaccines against Tuberculosis: Targeting the Disruption of Genes Encoding the Secretory Proteins of Mycobacteria

Tuberculosis (TB), a chronic infectious disease affecting humans, causes over 1.3 million deaths per year throughout the world. The current preventive vaccine BCG provides protection against childhood TB, but it fails to protect against pulmonary TB. Multiple candidates have been evaluated to either replace or boost the efficacy of the BCG vaccine, including subunit protein, DNA, virus vector-based vaccines, etc., most of which provide only short-term immunity. Several live attenuated vaccines derived from Mycobacterium tuberculosis (Mtb) and BCG have also been developed to induce long-term immunity. Since Mtb mediates its virulence through multiple secreted proteins, these proteins have been targeted to produce attenuated but immunogenic vaccines. In this review, we discuss the characteristics and prospects of live attenuated vaccines generated by targeting the disruption of the genes encoding secretory mycobacterial proteins.

MptpB Inhibitor Improves the Action of Antibiotics against Mycobacterium tuberculosis and Nontuberculous Mycobacterium avium Infections

Treatment of Mycobacterium tuberculosis and Mycobacterium avium infections requires multiple drugs for long time periods. Mycobacterium protein-tyrosine-phosphatase B (MptpB) is a key M. tuberculosis virulence factor that subverts host antimicrobial activity to promote intracellular survival. Inhibition of MptpB reduces the infection burden in vivo and offers new opportunities to improve current treatments. Here, we demonstrate that M. avium produces an MptpB orthologue and that the MptpB inhibitor C13 reduces the M. avium infection burden in macrophages. Combining C13 with the antibiotics rifampicin or bedaquiline showed an additive effect, reducing intracellular infection of both M. tuberculosis and M. avium by 50%, compared to monotreatment with antibiotics alone. This additive effect was not observed with pretomanid. Combining C13 with the minor groove-binding compounds S-MGB-362 and S-MGB-363 also reduced the M. tuberculosis intracellular burden. Similar additive effects of C13 and antibiotics were confirmed in vivo using Galleria mellonella infections. We demonstrate that the reduced mycobacterial burden in macrophages observed with C13 treatments is due to the increased trafficking to lysosomes.

The BCG Moreau Vaccine Upregulates In Vitro the Expression of TLR4, B7-1, Dectin-1 and EP2 on Human Monocytes

Background: Tuberculosis (TB) is currently the second greatest killer worldwide and is caused by a single infectious agent. Since Bacillus Calmette−Guérin (BCG) is the only vaccine currently in use against TB, studies addressing the protective role of BCG in the context of inducible surface biomarkers are urgently required for TB control. Methods: In this study, groups of HIV-negative adult healthy donors (HD; n = 22) and neonate samples (UCB; n = 48) were voluntarily enrolled. The BCG Moreau strain was used for the in vitro mononuclear cell infections. Subsequently, phenotyping tools were used for surface biomarker detection. Monocytes were assayed for TLR4, B7-1, Dectin-1, EP2, and TIM-3 expression levels. Results: At 48 h, the BCG Moreau induced the highest TLR4, B7-1, and Dectin-1 levels in the HD group only (p-value < 0.05). TIM-3 expression failed to be modulated after BCG infection. At 72 h, BCG Moreau equally induced the highest EP2 levels in the HD group (p-value < 0.005), and higher levels were also found in HD when compared with the UCB group (p-value < 0.05). Conclusions: This study uncovers critical roles for biomarkers after the instruction of host monocyte activation patterns. Understanding the regulation of human innate immune responses is critical for vaccine development and for treating infectious diseases.

Virulence-Associated Secretion in Mycobacterium abscessus

Non-tuberculous mycobacteria (NTM) are a heterogeneous group of originally environmental organi3sms, increasingly recognized as pathogens with rising prevalence worldwide. Knowledge of NTM’s mechanisms of virulence is lacking, as molecular research of these bacteria is challenging, sometimes more than that of M. tuberculosis (Mtb), and far less resources are allocated to their investigation. While some of the virulence mechanisms are common to several mycobacteria including Mtb, others NTM species-specific. Among NTMs, Mycobacterium abscessus (Mabs) causes some of the most severe and difficult to treat infections, especially chronic pulmonary infections. Mabs survives and proliferates intracellularly by circumventing host defenses, using multiple mechanisms, many of which remain poorly characterized. Some of these immune-evasion mechanisms are also found in Mtb, including phagosome pore formation, inhibition of phagosome maturation, cytokine response interference and apoptosis delay. While much is known of the role of Mtb-secreted effector molecules in mediating the manipulation of the host response, far less is known of the secreted effector molecules in Mabs. In this review, we briefly summarize the knowledge of secreted effectors in Mtb (such as ESX secretion, SecA2, TAT and others), and draw the parallel pathways in Mabs. We also describe pathways that are unique to Mabs, differentiating it from Mtb. This review will assist researchers interested in virulence-associated secretion in Mabs by providing the knowledge base and framework for their studies.

Antibody-Mediated LILRB2-Receptor Antagonism Induces Human Myeloid-Derived Suppressor Cells to Kill Mycobacterium tuberculosis

Tuberculosis is a leading cause of death in mankind due to infectious agents, and Mycobacterium tuberculosis (Mtb) infects and survives in macrophages (MФs). Although MФs are a major niche, myeloid-derived suppressor cells (MDSCs) are an alternative site for pathogen persistence. Both MФs and MDSCs express varying levels of leukocyte immunoglobulin-like receptor B (LILRB), which regulate the myeloid cell suppressive function. Herein, we demonstrate that antagonism of LILRB2 by a monoclonal antibody (mab) induced a switch of human MDSCs towards an M1-macrophage phenotype, increasing the killing of intracellular Mtb. Mab-mediated antagonism of LILRB2 alone and its combination with a pharmacological blockade of SHP1/2 phosphatase increased proinflammatory cytokine responses and phosphorylation of ERK1/2, p38 MAPK, and NF-kB in Mtb-infected MDSCs. LILRB2 antagonism also upregulated anti-mycobacterial iNOS gene expression and an increase in both nitric oxide and reactive oxygen species synthesis. Because genes associated with the anti-mycobacterial function of M1-MФs were enhanced in MDSCs following mab treatment, we propose that LILRB2 antagonism reprograms MDSCs from an immunosuppressive state towards a pro-inflammatory phenotype that kills Mtb. LILRB2 is therefore a novel therapeutic target for eradicating Mtb in MDSCs.

Human M1 macrophages express unique innate immune response genes after mycobacterial infection to defend against tuberculosis

Mycobacterium tuberculosis (Mtb) is responsible for approximately 1.5 million deaths each year. Though 10% of patients develop tuberculosis (TB) after infection, 90% of these infections are latent. Further, mice are nearly uniformly susceptible to Mtb but their M1-polarized macrophages (M1-MΦs) can inhibit Mtb in vitro, suggesting that M1-MΦs may be able to regulate anti-TB immunity. We sought to determine whether human MΦ heterogeneity contributes to TB immunity. Here we show that IFN-γ-programmed M1-MΦs degrade Mtb through increased expression of innate immunity regulatory genes (Inregs). In contrast, IL-4-programmed M2-polarized MΦs (M2-MΦs) are permissive for Mtb proliferation and exhibit reduced Inregs expression. M1-MΦs and M2-MΦs express pro- and anti-inflammatory cytokine-chemokines, respectively, and M1-MΦs show nitric oxide and autophagy-dependent degradation of Mtb, leading to increased antigen presentation to T cells through an ATG-RAB7-cathepsin pathway. Despite Mtb infection, M1-MΦs show increased histone acetylation at the ATG5 promoter and pro-autophagy phenotypes, while increased histone deacetylases lead to decreased autophagy in M2-MΦs. Finally, Mtb-infected neonatal macaques express human Inregs in their lymph nodes and macrophages, suggesting that M1 and M2 phenotypes can mediate immunity to TB in both humans and macaques. We conclude that human MФ subsets show unique patterns of gene expression that enable differential control of TB after infection. These genes could serve as targets for diagnosis and immunotherapy of TB.

Enterococcus faecalis alters endo-lysosomal trafficking to replicate and persist within mammalian cells

Enterococcus faecalis is a frequent opportunistic pathogen of wounds, whose infections are associated with biofilm formation, persistence, and recalcitrance toward treatment. We have previously shown that E. faecalis wound infection persists for at least 7 days. Here we report that viable E. faecalis are present within both immune and non-immune cells at the wound site up to 5 days after infection, raising the prospect that intracellular persistence contributes to chronic E. faecalis infection. Using in vitro keratinocyte and macrophage infection models, we show that E. faecalis becomes internalized and a subpopulation of bacteria can survive and replicate intracellularly. E. faecalis are internalized into keratinocytes primarily via macropinocytosis into single membrane-bound compartments and can persist in late endosomes up to 24 h after infection in the absence of colocalization with the lysosomal protease Cathepsin D or apparent fusion with the lysosome, suggesting that E. faecalis blocks endosomal maturation. Indeed, intracellular E. faecalis infection results in heterotypic intracellular trafficking with partial or absent labelling of E. faecalis-containing compartments with Rab5 and Rab7, small GTPases required for the endosome-lysosome trafficking. In addition, E. faecalis infection results in marked reduction of Rab5 and Rab7 protein levels which may also contribute to attenuated Rab incorporation into E. faecalis-containing compartments. Finally, we demonstrate that intracellular E. faecalis derived from infected keratinocytes are significantly more efficient in reinfecting new keratinocytes. Together, these data suggest that intracellular proliferation of E. faecalis may contribute to its persistence in the face of a robust immune response, providing a primed reservoir of bacteria for subsequent reinfection.

Enterococcus faecalis is often isolated from chronic wounds. Prior to this study, E. faecalis has been observed within different cell types, suggesting that it can successfully colonize intracellular spaces. However, to date, little is known about the mechanisms for E. faecalis intracellular survival. Here, we describe key features of the intracellular lifestyle of E. faecalis. We show that E. faecalis exists in an intracellular state within immune cells and non-immune cells during mammalian wound infection. We show that E. faecalis can survive and replicate inside keratinocytes and macrophages, and intracellularly replicating E. faecalis are primed to more efficiently cause reinfection, potentially contributing to chronic or persistent infections. To establish this intracellular lifestyle, E. faecalis is taken up by keratinocytes primarily via macropinocytosis, whereupon it manipulates the endosomal pathway and expression of trafficking molecules required for endo-lysosomal fusion, enabling E. faecalis to avoid lysosomal degradation and consequent death. These results advance our understanding of E. faecalis pathogenesis, demonstrating mechanistically how this classic extracellular pathogen can co-opt host cells for intracellular persistence, and highlight the heterogeneity of mechanisms bacteria can use to avoid host-mediated killing.

Lipid larceny: channelizing host lipids for establishing successful pathogenesis by bacteria

Lipids are complex organic compounds made up of carbon, oxygen, and hydrogen. These play a diverse and intricate role in cellular processes like membrane trafficking, protein sorting, signal transduction, and bacterial infections. Both Gram-positive bacteria (Staphylococcus sp., Listeria monocytogenes, etc.) and Gram-negative bacteria (Chlamydia sp., Salmonella sp., E. coli, etc.) can hijack the various host-lipids and utilize them structurally as well as functionally to mount a successful infection. The pathogens can deploy with various arsenals to exploit host membrane lipids and lipid-associated receptors as an attachment for toxins' landing or facilitate their entry into the host cellular niche. Bacterial species like Mycobacterium sp. can also modulate the host lipid metabolism to fetch its carbon source from the host. The sequential conversion of host membrane lipids into arachidonic acid and prostaglandin E2 due to increased activity of cPLA-2 and COX-2 upon bacterial infection creates immunosuppressive conditions and facilitates the intracellular growth and proliferation of bacteria. However, lipids' more debatable role is that they can also be a blessing in disguise. Certain host-lipids, especially sphingolipids, have been shown to play a crucial antibacterial role and help the host in combating the infections. This review shed light on the detailed role of host lipids in bacterial infections and the current understanding of the lipid in therapeutics. We have also discussed potential prospects and the need of the hour to help us cope in this race against deadly pathogens and their rapidly evolving stealthy virulence strategies.

A recombinant bovine adenoviral mucosal vaccine expressing mycobacterial antigen-85B generates robust protection against tuberculosis in mice

Although the BCG vaccine offers partial protection, tuberculosis remains a leading cause of infectious disease death, killing ∼1.5 million people annually. We developed mucosal vaccines expressing the autophagy-inducing peptide C5 and mycobacterial Ag85B-p25 epitope using replication-defective human adenovirus (HAdv85C5) and bovine adenovirus (BAdv85C5) vectors. BAdv85C5-infected dendritic cells (DCs) expressed a robust transcriptome of genes regulating antigen processing compared to HAdv85C5-infected DCs. BAdv85C5-infected DCs showed enhanced galectin-3/8 and autophagy-dependent in vitro Ag85B-p25 epitope presentation to CD4 T cells. BCG-vaccinated mice were intranasally boosted using HAdv85C5 or BAdv85C5 followed by infection using aerosolized Mycobacterium tuberculosis (Mtb). BAdv85C5 protected mice against tuberculosis both as a booster after BCG vaccine (>1.4-log10 reduction in Mtb lung burden) and as a single intranasal dose (>0.5-log10 reduction). Protection was associated with robust CD4 and CD8 effector (TEM), central memory (TCM), and CD103+/CD69+ lung-resident memory (TRM) T cell expansion, revealing BAdv85C5 as a promising mucosal vaccine for tuberculosis.

Discovery of uncompetitive inhibitors of SapM that compromise intracellular survival of Mycobacterium tuberculosis

SapM is a secreted virulence factor from Mycobacterium tuberculosis critical for pathogen survival and persistence inside the host. Its full potential as a target for tuberculosis treatment has not yet been exploited because of the lack of potent inhibitors available. By screening over 1500 small molecules, we have identified new potent and selective inhibitors of SapM with an uncompetitive mechanism of inhibition. The best inhibitors share a trihydroxy-benzene moiety essential for activity. Importantly, the inhibitors significantly reduce mycobacterial burden in infected human macrophages at 1 µM, and they are selective with respect to other mycobacterial and human phosphatases. The best inhibitor also reduces intracellular burden of Francisella tularensis, which secretes the virulence factor AcpA, a homologue of SapM, with the same mechanism of catalysis and inhibition. Our findings demonstrate that inhibition of SapM with small molecule inhibitors is efficient in reducing intracellular mycobacterial survival in host macrophages and confirm SapM as a potential therapeutic target. These initial compounds have favourable physico-chemical properties and provide a basis for exploration towards the development of new tuberculosis treatments. The efficacy of a SapM inhibitor in reducing Francisella tularensis intracellular burden suggests the potential for developing broad-spectrum antivirulence agents to treat microbial infections.

From infection niche to therapeutic target: the intracellular lifestyle of Mycobacterium tuberculosis

Mycobacterium tuberculosis (Mtb) is an obligate human pathogen killing millions of people annually. Treatment for tuberculosis is lengthy and complicated, involving multiple drugs and often resulting in serious side effects and non-compliance. Mtb has developed numerous complex mechanisms enabling it to not only survive but replicate inside professional phagocytes. These mechanisms include, among others, overcoming the phagosome maturation process, inhibiting the acidification of the phagosome and inhibiting apoptosis. Within the past decade, technologies have been developed that enable a more accurate understanding of Mtb physiology within its intracellular niche, paving the way for more clinically relevant drug-development programmes. Here we review the molecular biology of Mtb pathogenesis offering a unique perspective on the use and development of therapies that target Mtb during its intracellular life stage.

Host Cell Targets of Released Lipid and Secreted Protein Effectors of Mycobacterium tuberculosis

Mycobacterium tuberculosis (Mtb) is a very successful pathogen, strictly adapted to humans and the cause of tuberculosis. Its success is associated with its ability to inhibit host cell intrinsic immune responses by using an arsenal of virulence factors of different nature. It has evolved to synthesize a series of complex lipids which form an outer membrane and may also be released to enter host cell membranes. In addition, secreted protein effectors of Mtb are entering the host cell cytosol to interact with host cell proteins. We briefly discuss the current model, involving the ESX-1 type seven secretion system and the Mtb lipid phthiocerol dimycoserosate (PDIM), of how Mtb creates pores in the phagosomal membrane to allow Mtb proteins to access to the host cell cytosol. We provide an exhaustive list of Mtb secreted proteins that have effector functions. They modify (mostly inhibit but sometimes activate) host cell pathways such as: phagosome maturation, cell death, cytokine response, xenophagy, reactive oxygen species (ROS) response via NADPH oxidase 2 (NOX2), nitric oxide (NO) response via NO Synthase 2 (NOS2) and antigen presentation via MHC class I and class II molecules. We discuss the host cell targets for each lipid and protein effector and the importance of the Mtb effector for virulence of the bacterium.

Expression and purification of soluble recombinant SapM from Mycobacterium tuberculosis

SapM from Mycobacterium tuberculosis is a secreted phosphatase critical for pathogen survival inside the host, representing an attractive target for the development of anti-tuberculosis drugs. The main limitation to biochemical and structural studies of SapM has been the lack of a suitable protocol to produce soluble recombinant protein. The aim of the present work was to produce SapM in Escherichia coli in a soluble and catalytically active form. We describe here the construct design, expression and purification of soluble SapM using Sarkosyl as a solubility-enhancing agent and auto-induction media. We demonstrate that solubilisation of the recombinant protein with Sarkosyl, and further purification, yields a catalytically active enzyme with high purity and monodisperse. The identity and molecular weight of the recombinant SapM was confirmed by mass spectrometry analyses, and we provide evidence that SapM behaves as a monomer in solution. Overall, this work lays the foundation for further studies to exploit SapM as a drug target, and provides a protocol for producing active and soluble recombinant enzymes that are hard to solubilise in E. coli.

An autophagy-inducing and TLR-2 activating BCG vaccine induces a robust protection against tuberculosis in mice

Mycobacterium bovis BCG is widely used as a vaccine against tuberculosis due to M. tuberculosis (Mtb), which kills millions of people each year. BCG variably protects children, but not adults against tuberculosis. BCG evades phagosome maturation, autophagy, and reduces MHC-II expression of antigen-presenting cells (APCs) affecting T-cell activation. To bypass these defects, an autophagy-inducing, TLR-2 activating C5 peptide from Mtb-derived CFP-10 protein was overexpressed in BCG in combination with Ag85B. Recombinant BCG85C5 induced a robust MHC-II-dependent antigen presentation to CD4 T cells in vitro, and elicited stronger TH1 cytokines (IL-12, IL-1β, and TNFα) from APCs of C57Bl/6 mice increasing phosphorylation of p38MAPK and ERK. BCG85C5 also enhanced MHC-II surface expression of MΦs by inhibiting MARCH1 ubiquitin ligase that degrades MHC-II. BCG85C5 infected APCs from MyD88 or TLR-2 knockout mice showed decreased antigen presentation. Furthermore, BCG85C5 induced LC3-dependent autophagy in macrophages increasing antigen presentation. Consistent with in vitro effects, BCG85C5 markedly expanded both effector and central memory T cells in C57Bl/6 mice protecting them against both primary aerosol infection with Mtb and reinfection, but was less effective among TLR-2 knockout mice. Thus, BCG85C5 induces stronger and longer lasting immunity, and is better than BCG against tuberculosis of mice.

Mechanism of catalysis and inhibition of Mycobacterium tuberculosis SapM, implications for the development of novel antivirulence drugs

Mycobacterium tuberculosis (Mtb) SapM is a secreted virulence factor critical for intracellular survival of the pathogen. The role of SapM in phagosome maturation arrest in host macrophages suggests its potential as a drug target to assist in the clearance of tuberculosis infection. However, the mechanism of action of SapM at the molecular level remains unknown. In this study, we provide new insights into the mechanism of catalysis, substrate specificity and inhibition of SapM, and we identify the critical residues for catalysis and substrate binding. Our findings demonstrate that SapM is an atypical monoester alkaline phosphatase, with a serine-based mechanism of catalysis probably metal-dependent. Particularly relevant to SapM function and pathogenesis, is its activity towards PI(4,5)P₂ and PI3P, two phosphoinositides that function at the early stages of microbial phagocytosis and phagosome formation. This suggests that SapM may have a pleiotropic role with a wider importance on Mtb infection than initially thought. Finally, we have identified two inhibitors of SapM, L-ascorbic acid and 2-phospho-L-ascorbic, which define two different mechanisms by which the catalytic activity of this phosphatase could be regulated. Critically, we demonstrate that 2-phospho-L-ascorbic reduces mycobacterial survival in macrophage infections, hence confirming the potential of SapM as a therapeutic drug target.

SapM mutation to improve the BCG vaccine: Genomic, transcriptomic and preclinical safety characterization

The Mycobacterium bovis Bacille Calmette Guérin (BCG) vaccine shows variable efficacy in protection against adult tuberculosis (TB). Earlier, we have described a BCG mutant vaccine with a transposon insertion in the gene coding for the secreted acid phosphatase SapM, which led to enhanced long-term survival of vaccinated mice challenged with TB infection. To facilitate development of this mutation as part of a future improved live attenuated TB vaccine, we have now characterized the genome and transcriptome of this sapM::Tn mutant versus parental BCG Pasteur. Furthermore, we show that the sapM::Tn mutant had an equal low pathogenicity as WT BCG upon intravenous administration to immunocompromised SCID mice, passing this important safety test. Subsequently, we investigated the clearance of this improved vaccine strain following vaccination and found a more effective innate immune control over the sapM::Tn vaccine bacteria as compared to WT BCG. This leads to a fast contraction of IFNγ producing Th1 and Tc1 cells after sapM::Tn BCG vaccination. These findings corroborate that a live attenuated vaccine that affords improved long-term survival upon TB infection can be obtained by a mutation that further attenuates BCG. These findings suggest that an analysis of the effectiveness of innate immune control of the vaccine bacteria could be instructive also for other live attenuated TB vaccines that are currently under development, and encourage further studies of SapM mutation as a strategy in developing a more protective live attenuated TB vaccine.

Enhanced delivery of Mycobacterium tuberculosis antigens to antigen presenting cells using RVG peptide

Among the various strategies to improve vaccines against infectious diseases, targeting of antigens to dendritic cells (DCs), which are professional antigen presenting cells (APCs), has received increased attention in recent years. Here, we investigated whether a synthetic peptide region named RVG, originated from Rabies Virus Glycoprotein that binds to the α-7 subunit of the nicotinic acetylcholine receptors (AchR-α7) of APCs, could be used for the delivery of Mycobacterium tuberculosis (Mtb) peptide antigens to DCs and macrophages. Mouse bone marrow derived DCs (BMDCs) and human THP-1 macrophages stimulated with RVG fused peptide epitopes 85B₂₄₁ and 85B₉₆ (represent Ag85B_241-256 and Ag85B_96-111, respectively) from antigen 85B (Ag85B) of Mtb showed enhanced antigen presentation as compared to unfused peptide epitopes and BCG. Further, BMDCs stimulated with RVG fused 85B₂₄₁ showed higher levels of IL-12 positive cells. Consistent with in vitro data, splenocytes of mice immunized with RVG-85B₂₄₁ showed increased number of antigen specific IFN-γ, IL-2, and TNF-α producing cells in relation to splenocytes from mice immunized with 85B₂₄₁ alone. These results suggest that RVG may be a promising tool to develop effective alternate vaccines against tuberculosis (TB).

Tuberculosis and the art of macrophage manipulation

Macrophages are first-line responders against microbes. The success of Mycobacterium tuberculosis (Mtb) rests upon its ability to convert these antimicrobial cells into a permissive cellular niche. This is a remarkable accomplishment, as the antimicrobial arsenal of macrophages is extensive. Normally bacteria are delivered to an acidic, degradative lysosome through one of several trafficking pathways, including LC3-associated phagocytosis (LAP) and autophagy. Once phagocytozed, the bacilli are subjected to reactive oxygen and nitrogen species, and they induce the expression of proinflammatory cytokines, which serve to augment host responses. However, Mtb hijacks these host defense mechanisms, manipulating host cellular trafficking, innate immune responses, and cell death pathways to its benefit. The complex series of measures and countermeasures between host and pathogen ultimately determines the outcome of infection. In this review, we focus on the diverse effectors that Mtb uses in its multipronged effort to subvert the innate immune responses of macrophages. We highlight recent advances in understanding the molecular interface of the Mtb-macrophage interaction.

Ehrlichia type IV secretion system effector Etf-2 binds to active RAB5 and delays endosome maturation

Ehrlichia chaffeensis, an obligatory intracellular bacterium, infects monocytes/macrophages by sequestering a regulator of endosomal traffic, the small GTPase RAB5, on its membrane-bound inclusions to avoid routing to host-cell phagolysosomes. How RAB5 is sequestered on ehrlichial inclusions is poorly understood, however. We found that native Ehrlichia translocated factor-2 (Etf-2), a previously predicted effector of the Ehrlichia type IV secretion system, and recombinant Etf-2 (cloned into the Ehrlichia genome) are secreted into the host-cell cytoplasm and localize to ehrlichial inclusions. Ectopically expressed Etf-2-GFP also localized to inclusions and membranes of early endosomes marked with RAB5 and interacted with GTP-bound RAB5 but not with a GDP-bound RAB5. Etf-2, although lacking a RAB GTPase-activating protein (GAP) Tre2-Bub2-Cdc16 (TBC) domain, contains two conserved TBC domain motifs, namely an Arg finger and a Gln finger, and site-directed mutagenesis revealed that both Arg188 and Gln245 are required for Etf-2 localization to early endosomes. The yeast two-hybrid assay and microscale thermophoresis revealed that Etf-2 binds tightly to GTP-bound RAB5 but not to GDP-bound RAB5. However, Etf-2 lacks RAB5-specific GAP activity. Etf-2 localized to bead-containing phagosomes as well as endosomes containing beads coated with the C-terminal fragment of EtpE (entry-triggering protein of Ehrlichia), an Ehrlichia outer-membrane invasin, and significantly delayed RAB5 dissociation from and RAB7 localization to phagosomes/endosomes and RABGAP5 localization to endosomes. Thus, binding of Etf-2 to RAB5-GTP appears to delay RAB5 inactivation by impeding RABGAP5 localization to endosomes. This suggests a unique mechanism by which RAB5 is sequestered on ehrlichial inclusions to benefit bacterial survival and replication.

The patterns of in vitro cell-death and inflammatory cytokines induced by distinct BCG vaccine strains are differentially induced in human mononuclear cells

Tuberculosis (TB) remains among the world's leading cause of mortality. For its control, studies of TB vaccines are needed. Since live-attenuated Mycobacterium bovis bacillus Calmette-Guérin (BCG) is the only TB vaccine currently in use, studies on the protective role of BCG are required. In this study, we analyzed host cells purified directly from whole blood of human immunodeficiency virus (HIV)-negative volunteers, comprising adult healthy donors (HD) and neonates (umbilical cord bloods, UCB), with the aim to directly compare in vitro immune responses with distinct BCG strains in human mononuclear cells. The Moreau, Pasteur, and Danish BCG strains were used to infect mononuclear cells in vitro for 48 h; bacilli viability and cell-death were subsequently detected by flow cytometry. In addition, cell culture supernatants were used in cytokine detection assays. Overall, the Moreau BCG strain induced higher levels of apoptosis than the Pasteur and Danish BCG strains in both the HD and UCB groups (p-value < 0.05), and a human monocytic cell-line mirrored those cell-death patterns after BCG infection. The Moreau BCG strain, exclusively, induced Th1 cytokines at the highest levels in cells from adults (p-value < 0.05) when compared with both Pasteur and Danish BCG strains, whereas TGF-β1 levels were reduced significantly (p-value < 0.01) in the HD group when cells were infected with the Moreau BCG vaccine. As expected, eight out of 22 pro-inflammatory cytokines were secreted at significant levels (p-value < 0.05) above the baseline rates in all BCG-infected cell cultures, in the HD group only. When analyzing these results, we excluded confounding factors related to storage and viability of the BCG strains used. These findings suggest that Moreau BCG is a more potent immunostimulating agent than the Pasteur and Danish BCG strains. Clinical trials will be needed to confirm these findings.

Mycobacterium tuberculosis EsxH inhibits ESCRT-dependent CD4+ T-cell activation

Mycobacterium tuberculosis (Mtb) establishes a persistent infection, despite inducing antigen-specific T-cell responses. Although T cells arrive at the site of infection, they do not provide sterilizing immunity. The molecular basis of how Mtb impairs T-cell function is not clear. Mtb has been reported to block major histocompatibility complex class II (MHC-II) antigen presentation; however, no bacterial effector or host-cell target mediating this effect has been identified. We recently found that Mtb EsxH, which is secreted by the Esx-3 type VII secretion system, directly inhibits the endosomal sorting complex required for transport (ESCRT) machinery. Here, we showed that ESCRT is required for optimal antigen processing; correspondingly, overexpression and loss-of-function studies demonstrated that EsxH inhibited the ability of macrophages and dendritic cells to activate Mtb antigen-specific CD4⁺ T cells. Compared with the wild-type strain, the esxH-deficient strain induced fivefold more antigen-specific CD4⁺ T-cell proliferation in the mediastinal lymph nodes of mice. We also found that EsxH undermined the ability of effector CD4⁺ T cells to recognize infected macrophages and clear Mtb. These results provide a molecular explanation for how Mtb impairs the adaptive immune response.

Bacterial Virulence Factors: Secreted for Survival

Virulence is described as an ability of an organism to infect the host and cause a disease. Virulence factors are the molecules that assist the bacterium colonize the host at the cellular level. These factors are either secretory, membrane associated or cytosolic in nature. The cytosolic factors facilitate the bacterium to undergo quick adaptive-metabolic, physiological and morphological shifts. The membrane associated virulence factors aid the bacterium in adhesion and evasion of the host cell. The secretory factors are important components of bacterial armoury which help the bacterium wade through the innate and adaptive immune response mounted within the host. In extracellular pathogens, the secretory virulence factors act synergistically to kill the host cells. In this review, we revisit the role of some of the secreted virulence factors of two human pathogens: Mycobacterium tuberculosis-an intracellular pathogen and Bacillus anthracis-an extracellular pathogen. The advances in research on the role of secretory factors of these pathogens during infection are discussed.

Crucial requirement for standardization during the development of novel recombinant BCG vaccines: Does the corresponding substrain background matter?

The Bacillus Calmette-Guerin (BCG) vaccine is not a single organism, but consists of substrains that vary in genotypes and phenotypes. Actually, BCG is the common name given to a family of vaccines created in 1921 by the in vitro attenuation of a virulent Mycobacterium bovis in France. Even nearly a century of use, the BCG vaccine lingers generating confusion and debate due to its diversity and failure to protect against tuberculosis (TB). That is probably owing to the enduring lack of standardization during production, distribution and administration procedures. Since the 1940s, substantial sequence modifications among the BCG substrains have been described. To increase the level of complexity, even though that the prolific generation of recombinant BCG vaccines has been promising, the relationships between those candidates used in current clinical trials and their parental substrains are either unsatisfactorily connected or have been never fully delineated. Consequently, the use of the most protective BCG substrain as the background or platform in the development of all recombinant BCG vaccine candidates has not been standardized. In order to schematize and to clarify the subject regarding substrains commonly used to generate those novel vaccines, a sequential emergence of the parental BCG vaccine substrains and their matching recombinant ones, if any, was built. Hence, for a total of 24 BCG substrains currently in circulation worldwide, 9 have been used to sustain one or more genetic modifications, resulting in around 21 novel recombinant BCG vaccines. Although this is a remarkable success, only 2 out of the 21 recombinant BCG substrains harbor a background representative of the most immunogenic group. Systematizing the novel BCG vaccines and their parental strains may facilitate our understanding of protection provided by BCG immunizations.

Deciphering the roles of phosphoinositide lipids in phagolysosome biogenesis

Professional phagocytes engulf microbial invaders into plasma membrane-derived phagosomes. These mature into microbicidal phagolysosomes, leading to killing of the ingested microbe. Phagosome maturation involves sequential fusion of the phagosome with early endosomes, late endosomes, and the main degradative compartments in cells, lysosomes. Some bacterial pathogens manipulate the phosphoinositide (PIP) composition of phagosome membranes and are not delivered to phagolysosomes, pointing at a role of PIPs in phagosome maturation. This hypothesis is supported by comprehensive microscopic studies. Recently, cell-free reconstitution of fusion between phagosomes and endo(lyso)somes identified phosphatidylinositol 4-phosphate [PI(4)P] and phosphatidylinositol 3-phosphate [PI(3)P] as key regulators of phagolysosome biogenesis. Here, we describe the emerging roles of PIPs in phagosome maturation and we present tools to study PIP involvement in phagosome trafficking using intact cells or purified compartments.

Current efforts and future prospects in the development of live mycobacteria as vaccines

The development of more effective vaccines against Mycobacterium tuberculosis (Mtb) remains a major goal in the effort to reduce the enormous global burden of disease caused by this pathogen. Whole-cell vaccines based on live mycobacteria with attenuated virulence represent an appealing approach, providing broad antigen exposure and intrinsic adjuvant properties to prime durable immune responses. However, designing vaccine strains with an optimal balance between attenuation and immunogenicity has proven to be extremely challenging. Recent basic and clinical research efforts have broadened our understanding of Mtb pathogenesis and created numerous new vaccine candidates that have been designed to overcome different aspects of immune evasion by Mtb. In this review, we provide an overview of the current efforts to create improved vaccines against tuberculosis based on modifications of live attenuated mycobacteria. In addition, we discuss the use of such vaccine strains as vectors for stimulating protective immunity against other infectious diseases and cancers.

A rational framework for evaluating the next generation of vaccines against Mycobacterium avium subspecies paratuberculosis

Since the early 1980s, several investigations have focused on developing a vaccine against Mycobacterium avium subspecies paratuberculosis (MAP), the causative agent of Johne's disease in cattle and sheep. These studies used whole-cell inactivated vaccines that have proven useful in limiting disease progression, but have not prevented infection. In contrast, modified live vaccines that invoke a Th1 type immune response, may improve protection against infection. Spurred by recent advances in the ability to create defined knockouts in MAP, several independent laboratories have developed modified live vaccine candidates by transpositional mutation of virulence and metabolic genes in MAP. In order to accelerate the process of identification and comparative evaluation of the most promising modified live MAP vaccine candidates, members of a multi-institutional USDA-funded research consortium, the Johne's disease integrated program (JDIP), met to establish a standardized testing platform using agreed upon protocols. A total of 22 candidates vaccine strains developed in five independent laboratories in the United States and New Zealand voluntarily entered into a double blind stage gated trial pipeline. In Phase I, the survival characteristics of each candidate were determined in bovine macrophages. Attenuated strains moved to Phase II, where tissue colonization of C57/BL6 mice were evaluated in a challenge model. In Phase III, five promising candidates from Phase I and II were evaluated for their ability to reduce fecal shedding, tissue colonization and pathology in a baby goat challenge model. Formation of a multi-institutional consortium for vaccine strain evaluation has revealed insights for the implementation of vaccine trials for Johne's disease and other animal pathogens. We conclude by suggesting the best way forward based on this 3-phase trial experience and challenge the rationale for use of a macrophage-to-mouse-to native host pipeline for MAP vaccine development.

Secretory phosphatases deficient mutant of Mycobacterium tuberculosis imparts protection at the primary site of infection in guinea pigs

Background

The failure of Mycobacterium bovis Bacille Calmette-Guérin to impart satisfactory protection against adult pulmonary tuberculosis has necessitated the development of more effective TB vaccines. The assumption that the vaccine strain should be antigenically as similar as possible to the disease causing pathogen has led to the evaluation of M.tuberculosis mutants as candidate tuberculosis vaccines.

Methods/Principal Findings

In this study, we have generated a mutant of M.tuberculosis (Mtb∆mms) by disrupting 3 virulence genes encoding a mycobacterial secretory acid phosphatase (sapM) and two phosphotyrosine protein phosphatases (mptpA and mptpB) and have evaluated its protective efficacy in guinea pigs. We observed that Mtb∆mms was highly attenuated in THP-1 macrophages. Moreover, no bacilli were recovered from the lungs and spleens of guinea pigs after 10 weeks of Mtb∆mms inoculation, although, initially, the mutant exhibited some growth in the spleens. Subsequently, when Mtb∆mms was evaluated for its protective efficacy, we observed that similar to BCG vaccination, Mtb∆mms exhibited a significantly reduced CFU in the lungs of guinea pigs when compared with the unvaccinated animals at 4 weeks after challenge. In addition, our observations at 12 weeks post challenge demonstrated that Mtb∆mms exhibited a more sustainable and superior protection in lungs as compared to BCG. However, the mutant failed to control the hematogenous spread as the splenic bacillary load between Mtb∆mms vaccinated and sham immunized animals was not significantly different. The gross pathological observations and histopathological observations corroborated the bacterial findings. Inspite of disruption of phosphatase genes in MtbΔmms, the lipid profiles of M.tuberculosis and MtbΔmms were identical indicating thereby that the phenotype of the mutant was ascribed to the loss of phosphatase genes and the influence was not related to any alteration in the lipid composition.

Conclusions/Significance

This study highlights the importance of M.tuberculosis mutants in imparting protection against pulmonary TB.

Secreted acid phosphatase (SapM) of Mycobacterium tuberculosis is indispensable for arresting phagosomal maturation and growth of the pathogen in guinea pig tissues

Tuberculosis (TB) is responsible for nearly 1.4 million deaths globally every year and continues to remain a serious threat to human health. The problem is further complicated by the growing incidence of multidrug-resistant TB (MDR-TB) and extensively drug-resistant TB (XDR-TB), emphasizing the need for the development of new drugs against this disease. Phagosomal maturation arrest is an important strategy employed by Mycobacterium tuberculosis to evade the host immune system. Secretory acid phosphatase (SapM) of M.tuberculosis is known to dephosphorylate phosphotidylinositol 3-phosphate (PI3P) present on phagosomes. However, there have been divergent reports on the involvement of SapM in phagosomal maturation arrest in mycobacteria. This study was aimed at reascertaining the involvement of SapM in phagosomal maturation arrest in M.tuberculosis. Further, for the first time, we have also studied whether SapM is essential for the pathogenesis of M.tuberculosis. By deleting the sapM gene of M.tuberculosis, we demonstrate that MtbΔsapM is defective in the arrest of phagosomal maturation as well as for growth in human THP-1 macrophages. We further show that MtbΔsapM is severely attenuated for growth in the lungs and spleen of guinea pigs and has a significantly reduced ability to cause pathological damage in the host when compared with the parental strain. Also, the guinea pigs infected with MtbΔsapM exhibited a significantly enhanced survival when compared with M.tuberculosis infected animals. The importance of SapM in phagosomal maturation arrest as well as in the pathogenesis of M.tuberculosis establishes it as an attractive target for the development of new therapeutic molecules against tuberculosis.

Elimination of intracellularly residing Mycobacterium tuberculosis through targeting of host and bacterial signaling mechanisms

With more than 2 billion latently infected people, TB continues to represent a serious threat to human health. According to the WHO, 1.1 million people died from TB in 2010, which is equal to approximately 3000 deaths per day. The causative agent of the disease, Mycobacterium tuberculosis, is a highly successful pathogen having evolved remarkable strategies to persist within the host. Although normally, upon phagocytosis by macrophages, bacteria are readily eliminated by lysosomes, pathogenic mycobacteria actively prevent destruction within macrophages. The strategies that pathogenic mycobacteria apply range from releasing virulence factors to manipulating host molecules resulting in the modulation of host signal transduction pathways in order to sustain their viability within the infected host. Here, we analyze the current status of how a better understanding of both the bacterial and host factors involved in virulence can be used to develop drugs that may be helpful to curb the TB epidemic.

Current and novel approaches to vaccine development against tuberculosis

Antibiotics and vaccines are the two most successful medical countermeasures that humans have created against a number of pathogens. However a select few e.g., Mycobacterium tuberculosis (Mtb), the causative agent of tuberculosis (TB) have evaded eradication by vaccines and therapeutic approaches. TB is a global public health problem that kills 1.4 million people per year. The past decade has seen significant progress in developing new vaccine candidates, but the most fundamental questions in understanding disease progression and protective host responses that are responsible for controlling Mtb infection still remain poorly resolved. Current TB treatment requires intense chemotherapy with several antimicrobials, while the only approved vaccine is the classical viable whole-cell based Bacille-Calmette-Guerin (BCG) that protects children from severe forms of TB, but fails to protect adults. Taken together, there is a growing need to conduct basic and applied research to develop novel vaccine strategies against TB. This review is focused on the discussion surrounding current strategies and innovations being explored to discover new protective antigens, adjuvants, and delivery systems in the hopes of creating an efficacious TB vaccine.

Candida albicans induces arginine biosynthetic genes in response to host-derived reactive oxygen species

The interaction of Candida albicans with phagocytes of the host's innate immune system is highly dynamic, and its outcome directly impacts the progression of infection. While the switch to hyphal growth within the macrophage is the most obvious physiological response, much of the genetic response reflects nutrient starvation: translational repression and induction of alternative carbon metabolism. Changes in amino acid metabolism are not seen, with the striking exception of arginine biosynthesis, which is upregulated in its entirety during coculture with macrophages. Using single-cell reporters, we showed here that arginine biosynthetic genes are induced specifically in phagocytosed cells. This induction is lower in magnitude than during arginine starvation in vitro and is driven not by an arginine deficiency within the phagocyte but instead by exposure to reactive oxygen species (ROS). Curiously, these genes are induced in a narrow window of sublethal ROS concentrations. C. albicans cells phagocytosed by primary macrophages deficient in the gp91(phox) subunit of the phagocyte oxidase do not express the ARG pathway, indicating that the induction is dependent on the phagocyte oxidative burst. C. albicans arg pathway mutants are retarded in germ tube and hypha formation within macrophages but are not notably more sensitive to ROS. We also find that the ARG pathway is regulated not by the general amino acid control response but by transcriptional regulators similar to the Saccharomyces cerevisiae ArgR complex. In summary, phagocytosis induces this single amino acid biosynthetic pathway in an ROS-dependent manner.